1. Field of the Invention
The invention relates to an improvement in the treatment of hydrocarbon distillates, more particularly to an improved method of sweetening sour hydrocarbon distillates by oxidizing mercaptans in the distillate to disulfides in the presence of a phthalocyanine catalyst on a charcoal carrier in the presence of a basic medium and oxygen.
2. Prior Art
Sweetening of sour hydrocarbons is well known in the petroleum refining arts. Processes abound relating to the treatment of petroleum distillates such as sour gasoline, cracked gasoline, straight run gasoline, naphtha, jet fuel, kerosene, fuel oil, etc.
The prime offender in many hydrocarbon distillates is mercaptan sulfur, RSH. Mercaptan sulfur can be successfully removed by hydrotreating, using a catalyst containing Co, Mo, etc., on a carrier such as alumina, at high temperatures under high hydrogen pressures. This hydrotreating will convert mercaptan sulfur to H.sub.2 S which can be removed from normally liquid hydrocarbon fractions by distillation.
Hydrotreating is relatively expensive, and many petroleum products can contain relatively high sulfur levels, as long as the sulfur is not in the form of a mercaptan. The mercaptans are objectionable because of their strong odor, and because they are more corrosive. For many processes, it is sufficient if the mercaptans are converted to disulfides, RSSH, or RSSR.
A process for the fixed bed sweetening of hydrocarbon distillates is shown in U.S. Pat. No. 2,988,500 (Class 208-206), the teachings of which are incorporated by reference. In this patent, a novel catalyst was used to oxidize mercaptans to disulfides. The novel catalyst disclosed in this patent was cobalt phthalocyanine sulfonate composited with a charcoal carrier. A mixture of sour kerosene, aqueous NaOH solution, and air were passed over the catalyst to convert mercaptan sulfur to a level low enough that the kerosene product recovered would be doctor sweet. The treating reaction was effected in the presence of an alkaline reagent. The patentee taught that any suitable alkaline reagent could be used, and taught that the preferred reagents were sodium hydroxide and potassium hydroxide. Other reagents considered possible were aqueous solutions of lithium hydroxide, rubidium hydroxide, and cesium hydroxide.
Another treating process, inhibitor sweetening, was disclosed in U.S. Pat. No. 2,744,854 (Class 496-29), the teachings of which are incorporated by reference. The sweetening reaction was always accomplished in storage tanks, rather than in a reactor vessel. Thus, reaction times of several days would be necessary to complete the conversion of mercaptan sulfur to disulfides. There is extremely detailed and broad teaching in this patent as to the type of basic reagent which may be used to facilitate the sweetening reaction. Both organic and inorganic bases are taught, though from the examples, use of a phenylene diamine is preferred. Optionally, a metal chelate may be added to speed up the sweetening which occurs in the storage tank. In the specific teachings on basic compounds which may be used in addition to sodium hydroxide or potassium hydroxide, the patentee teaches over 50 different compounds and classes of compounds which serve as basic reagents.
Another inhibitor sweetening process is disclosed in U.S. Pat. No. 2,983,674 (Class 208-207), the teachings of which are incorporated by reference. This reference discloses that guanidines may be used in the inhibitor sweetening process to supplement the phenylene diamines used in this process. The number of guanidines disclosed is impressive going from column 2, line 70 to column 4, line 17. The patentee stated that no inoperable guanidine had been found and that all were operable. Many examples disclosed use of tetra-alkyl guanidines.
Although inhibitor sweetening and phthalocyanine catalyst oxidation both decrease the mercaptan content of a fuel, the means by which this is accomplished are different in the two cases. Consequently, those factors which control the process in one case cannot be considered as applicable to the other. The two processes must be considered as dissimilar.
The phthalocyanine catalyzed process carried out one reaction: the conversion of mercaptans to disulfides. This is accomplished by use of a chelated metal catalyst which, in commercial operation, is in a separate phase insoluble in the fuel. The reactions take place at the interface and consequently the process is susceptible to surface active ingredients.
In contrast, inhibitor sweetening involves several reactions, with disulfide formation accounting for, at most, two thirds of the mercaptan converted. At least one third of the mercaptan is consumed by interaction with olefins which must be present for sweetening to occur. These reactions involve species as intermediates called "free radicals" which are not observed in the phthalocyanine set of reactions. The inhibitor sweetening reactions generally are carried out in a single phase (hydrocarbon) with a catalyst, a specific type of organic polyamine, miscible in the hydrocarbon.
The two processes proceed under such different conditions, with different intermediates and with different products formed, that the two must be considered separate systems.
It is possible to pin-point the specific portions of the two processes at which the reactions with a basic material, such as a tetra-alkyl guanidine come into play. In case of phthalocyanine catalyzed oxidation, the function of the base is to convert the mercaptan to the corresponding anion: EQU RSH.fwdarw.RS
This is accomplished by such strong bases as sodium hydroxide and guanidine. A strong base is essential for the catalyst will bring the mercaptan in play only in the ionized or anion form (RS).
In case of inhibitor sweetening, the function of the base is to bring about the following reaction: EQU R'OOH+2RSH.fwdarw.R'OH+H.sub.2 O+RSSR
A hydroperoxide (R'OOH) of rather complicated structure is formed as intermediate, and this oxidant converts mercaptan into disulfide. The reaction requires the presence of a base, and there are a number available.
An inspection of the actions of the base in the two processes reveals that the functions are different. Consequently one cannot reliably predict the effect of a given base from one process to the other.
The substitution of one base for another in the two processes does not always work. For example, an organic amine (R.sub.3 N) is suitable to carry out inhibitor sweetening, perhaps not as well as use of sodium hydroxide but still acceptable. In fact, an amine is incorporated in a commercial product (UOP 5-S) to impart the basicity needed for inhibitor sweetening.
In contrast, an organic amine is not only ineffective but deleterious to the process with phthalocyanine catalyst. In other words, it is impossible to predict the effect in the phthalocyanine system from results from inhibitor sweetening.
Inhibitor sweetening can proceed with bases which are generally considered as "weak". Phthalocyanine reactions require a "strong" or "stronger" base. Since the amine R.sub.3 N is much "weaker" than a tetralkyl guanidine, the preceding observations can be explained.
In consideration of various bases, a strength is assigned to each base as a single attribute. This actually is not the case, for the basicity, and likewise acidity, is an intricate relationship of parts. For example, a concept has developed of "hard and soft" acids and bases which separates acids and bases into classes (R. G. Pearson, J. Chem. Educ., 45 581 (1968) and 45 643 (1968)). In other words, there is no one property of a base which carries over into all cases.
A similar conclusion is made with the Lewis definition of acids and bases, particularly the role of acids and bases as catalysts (see Kirk Othmer Encyclopedia of Chemical Technology, Second Edition, Vol. 1, pages 118-22).
Literature reports in general (for example, A. Frost and R. Pearson, Kinetics and Mechanism, John Wiley and Sons, Inc.) that it is difficult to correlate efficiencies of various bases for different reactions. The prediction of efficiencies should be even more laborious and uncertain.
A "weak" base is suitable for use in inhibitor sweetening; an amine R.sub.3 N is moderately effective. One would expect an ammonium hydroxide solution to be similarly effective; generally, this is not the case. One can develop a concept to predict basicity effects, but they generally do not have universal application.
In the phthalocyanine system, an ammonium hydroxide solution likewise is of such low effectiveness that it has no practical utility.
On the basis of the foregoing discussion one can conclude that the two mercaptan conversion processes, inhibitor sweetening and phthalocyanine catalyzed oxidation, are two dissimilar systems and bear little resemblance to each other. Consequently there is no basis to propose predictability as to the effect of a given alkaline material on the system.
The fixed bed sweetening process has enjoyed worldwide commercial success. Despite the great acceptance of fixed bed sweetening by refining industry, there are still a few areas in which attempts have been made to improve the process. Specifically, the practice of using aqueous sodium hydroxide solutions to provide the basic medium required for oxidizing mercaptans to disulfides has resulted in a caustic disposal problem. Eventually the caustic solution used in a fixed bed unit becomes unsuitable for further use. Most common reason for discarding of caustic solutions is that various toxins or catalyst poisons, generated by the oxidation reaction, accumulate in the caustic. Thus, for a number of reasons the caustic commonly used in fixed bed sweetening processes must be discarded. Although sodium hydroxide is a very inexpensive chemical to buy, it is becoming a relatively expensive chemical to throw away, because of concern about pollution.
Also of concern to refiners is the danger that some of the caustic solution will somehow find its way into the final product. For some uses, e.g., jet fuel, neither sodium hydroxide nor water may be tolerated in the product. Elaborate measures are taken to make sure that the kerosene product destined for use as jet fuel will not contain either water or NaOH. The solution commonly used is to water-wash the kerosene effluent from the fixed bed treating process to remove sodium hydroxide solution. The water-washed kerosene is then passed through a bed of salt, so that the salt will react with any water contained in the hydrocarbon, and from a brine which will remain behind. Finally, the kerosene is passed through a bed of clay or sand to remove the last traces of water or brine solution which may be in the product. Although effective, such elaborate measures add to the cost of treating and increase the capital expenditure required to build a plant for the treating of fuels where the presence of small amounts of aqueous sodium hydroxide solutions is objectionable.
Other problems which have been encountered in the fixed bed sweetening process are the occasional plugging of the catalyst bed due to formation of soaps. A number of hydrocarbon distillates contained naphthenic acids, and the naphthenic acids reacted with aqueous sodium hydroxide to form a soap which forms a gel with the hydrocarbon which in turn plugged the charcoal bed. It has been necessary to put in caustic prewashes to remove these naphthenic acids from feeds containing them, so that the feed to the fixed bed sweetening unit will be substantially free of naphthenic acids. The typical naphthenic acid prewash is a large vessel filled with a dilute solution of sodium hydroxide. While such a vessel is efficient, and relatively inexpensive, it still adds to the cost of operating a fixed bed treating process.
Because of these difficulties encountered with some feedstocks, and some product specifications, I tried to find some way to eliminate these problems entirely, rather than add on an extra step upstream or downstream of existing fixed bed treating units. My investigation showed that most of the problems were caused by either something in the feed reacting with the aqueous sodium hydroxide solution used as a basic medium, or caused by remnants of the basic medium appearing in the product. I discovered a replacement for the sodium hydroxide solutions currently used in fixed bed treating processes. The replacement provided a uniquely satisfactory substitute for customarily used basic solutions. The material I discovered was tetra-alkyl guanidines.